Autoacceleration

When the concentrations of monomers are high in solution or bulk polymerizations, typical autoaccelerations of the rates can be observed. This is known as the gel effect or as the Trammsdorff effect, or also, as the Norrish—Smith effect [66]. The effect has been explained as being caused by a decrease in the rate of termination due to increased viscosity of the medium. Termination is a reaction that requires two large polymer-radicals to come together and this can be impeded by viscosity. At the same time, in propagation the small molecules of the monomer can still diffuse for some time to the radical sites and feed the chain growth. 

One should not mistake an acceleration of the polymerization reaction due to a rise in the temperature under nonisothermal conditions for a true gel effect from a rise in viscosity. The gel effect can occur when the temperature of the reaction is kept constant. 

Deviations from ideal kinetics due to size-dependence and diffusion control of termination produce relatively weak effects at low conversions. However, at high conversions these effects are very signi cant in most radical polymerizations. Thus, instead of the reaction rate falling with time, as would be expected from Eq. (6.24) since the monomer and initiator concentrations decrease with conversion, an exact opposite behavior is observed in many polymerizations where the rate of polymerization increases with time. A typical example of this phenomenon is shown in Fig. 6.10 for the polymerization of methyl methacrylate in benzene solution at 50°C (Schulz and Haborth, 1948). An acceleration is observed at relatively high monomer concentrations and the curve for the pure monomer shows a drastic autoacceleration in the polymerization rate. This type of behavior observed under isothermal conditions is referred to as the gel effect. It is also known as the Tromsdorff effect or Norrish-Smith effect in honor of the early researchers in this eld. 

While termination involves reaction of two large polymer radicals, propagation involves the reaction of a large radical with small monomer molecules. High viscosity thus affects the termination reaction much more than the propagation reaction, that is, kt decreases much more than kp, the net result being that there is an increase in the ratio kpjk] in Eq. (6.24) and hence an increase in the rate of polymerization. As vinyl polymerizations are exothermic (see later), this increased rate can cause a temperature rise and faster initiator decomposition, leading nally to runaway reaction conditions. 

Problem 6.28 The bimolecular chain termination in free-radical polymerization is a diffusion-controlled reaction that can be treated as a three-stage process (North and Reid, 1963 Odian, 1991), described below. Stage 1. Translational diffusion of the centers of gravity of two macroradicals to such close proximity that certain segments of each chain can be considered to be in contact  

Stage 2. Segmental diffusion (movement of parts or segments of a polymer chain relative to its other parts) of the two chains bringing their radical ends suf ciently close for chemical reaction  

Assuming that diffusion is the rate-determining process for termination, obtain an expression for the rate of termination. Simplify the expression for two limiting situations of slow translational diffusion and slow 

Stage 1. Translational diffusion of the centers of gravity of two macroradicals to such close proximity that certain segments of each chain can be considered to be in contact  

The gel effect observed for higher monomer concentrations is generally attributed to a decrease of the termination rate constant kt due to increased viscosity at higher conversions. Theoretical considerations indicate that the rate constant for reaction between two radicals in low viscosity media (such as bulk monomer) would be very large, about 8x10 L/mol-s. Experiment- 

Problem 6.34 Chain termination is a diffusion-controlled reaction best described [50-52] as occurring by a three step process  

Step 1. Translational diffusion of two propagating radicals (i.e., movement of the whole radicals) until they are in close proximity to each other  

For the major duration of a chain polymerisation the reaction is first-order in monomer concentration. However, at high conversions of monomer to polymer using either undiluted monomer or concentrated solutions there is a significant 

The explanation for autoacceleration is as follows. As polymerisation proceeds there is an increase in the viscosity of the reaction mixture which reduces the mobility of the reacting species. Growing polymer molecules are more affected by this than either the molecules of monomer or the fragments arising from decomposition of the initiator. Hence termination reactions slow down and eventually stop, while initiation and propagation reactions still continue. Such a decrease in the rate of the termination steps thus leads to the observed increase in the overall rate of polymerisation. 

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Autoacceleration in the polymerization of MA poses a serious problem [21-23]. Saini et al. [24] attempted to polymerize MA by using /3-PCPY as the initiator with a view to minimize the difficulties experienced due to this phenomenon. The findings led to the conclusion that -PCPY can be used to obtain 19.5% conversion of MA without gelation due to autoacceleration, which is nearly double the conversion obtained by using the conventional free radical initiator (AIBN) in the same experimental conditions. 

Ylides can be used as initiators to get a polymethacrylate of Pn 571 at 14% conversion to minimize the autoacceleration effect. 

This model accounts for the coupling between molecular weight development and autoacceleration in R. However, two of the basic assumptions... 

The predominant mode of polymerization is in the interior of the particles and this leads to a reduction of macroradical mobility, usually referred to as radical occlusion, and a marked autoacceleration of the polymerization rate. 

Some typical examples of this autoacceleration are (Figure 5) Norrish and Smith ( 2) polymerized methyl methacrylate in bulk and in the presence of various precipitants and measured the polymerization rates dilatometrically. They determined that autoacceleration of the precipitation polymerizations was larger than that observed for the Trommsdorf effect in bulk polymerization. 

Finally, similar autoacceleration in the polymerization rate was reported by Crosato-Arnaldi, Gasparini and Talamini (18) for the bulk polymerization of vinyl chloride. 

Figure 5. Effect of autoacceleration on the precipitation polymerization of methyl methacrylate (2). The curves, from left to right, are for the diluents cyclohexane t-hutylsterate heptane and bulk.

The precipitation polymerization literature is reviewed with particular attention to the influence of particle formation and growth, autoaccelerating polymerization rates, and copolymer composition drift on polymer reactor design. 

Autoacceleration, Glass and Zutty (S) and Burnett and Melville 9) reported an increase in the rate and average degree of polymerization with increasing solution viscosity, heterogeneous conditions and chain coiling for free radical, vinyl polymerizations. Autoacceleration is also called Trommsdorff. (10) effect. 

Autoacceleration in the rate of polymerization occurs also with other monomers. It is far more marked with methyl acrylate or acrylic... 

In quest of an explanation for this phenomenon, one is led to conclude either that the combination of constants occurring in the rate equation (12) must undergo a large increase when autoacceleration occurs or that a totally different mechanism of polymerization must take over. We should obviously prefer the former alternative if it will lead to a satisfactory explanation of the facts. An increase in kdj seems unlikely autoacceleration is not a function of the initiator. This leaves us with the ratio which will be required to increase by... 

The above explanation of autoacceleration phenomena is supported by the manifold increase in the initial polymerization rate for methyl methacrylate which may be brought about by the addition of poly-(methyl methacrylate) or other polymers to the monomer.It finds further support in the suppression, or virtual elimination, of autoacceleration which has been observed when the molecular weight of the polymer is reduced by incorporating a chain transfer agent (see Sec. 2f), such as butyl mercaptan, with the monomer.Not only are the much shorter radical chains intrinsically more mobile, but the lower molecular weight of the polymer formed results in a viscosity at a given conversion which is lower by as much as several orders of magnitude. Both factors facilitate diffusion of the active centers and, hence, tend to eliminate the autoacceleration. Final and conclusive proof of the correctness of this explanation comes from measurements of the absolute values of individual rate constants (see p. 160), which show that the termination constant does indeed decrease a hundredfold or more in the autoacceleration phase of the polymerization, whereas kp remains constant within experimental error. 

The susceptibility of the polymerization of a given monomer to autoacceleration seems to depend primarily on the size of the polymer molecules produced. The high propagation and low termination constants for methyl acrylate as compared to those for other common monomers lead to an unusually large average degree of polymerization (>10 ), and this fact alone seems to account for the incidence of the decrease in A f at very low conversions in this case. 

Owing to the necessity for extrapolating measurements on methyl acrylate to a conversion of less than 1 percent in order to avoid the pronounced autoacceleration occurring with this monomer, the data are of lower accuracy than for most of the other monomers investigated. 

If converted into plots of fraction conversion versus time, these forms give rise to a characteristic S shape. These plots first rise, showing autoacceleration as the rate increases, then pass through an inflection point as the rate reaches a maximum, and finally taper off so that the fraction conversion approaches unity or its equilibrium value as the time approaches infinity. 

Provided that chemiluminescence intensity Iql is proportional to the rate of peroxyl radicals termination, that is Icl [PO ]2, which is often assumed in the literature, chemiluminescence intensity should achieve some quasi-stationary level when hydroperoxide concentration becomes stationary and its decay should correspond to consumption of oxidizable groups, PH, in a polymer. At the same time, the chemiluminometric curves of type (a), which are typical with an autoaccelerating increase of the light emission (Figure 4) are relevant for... 

Comparison of chemiluminescence isothermal runs with oxygen uptake and DSC measurements has been at the centre of interest since practical industrial applications of the chemiluminescence method were attempted. It is a fact that the best comparison may be achieved when studying polymers that give a distinct induction time of oxidation typical for autoaccelerating curves of a stepwise developing oxidation. This is the particular case of polyolefins, polydienes and polyamides. The theoretical justification for the search of a mutual relationship between the oxidation runs found by the various methods follows directly from the kinetic analysis of the Bolland-Gee scheme of polymer oxidation. 

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